AN1673 datasheet(10/24 Pages) STMICROELECTRONICS

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AN1673 APPLICATION NOTE

10/24

Figure 11. Resonance's LCL-Resonant Converter

In the diagram at Fig.11, it can be observed the net circuit (seen from the OUT node) driven by the half

bridge totem pole. The circuit path consists in the series of a determine inductors (L_RES1), the primary of

the transformer (LMag. // Load) and the resonant capacitor (CRES). We call the series of the L_RES1 and

the Magnetizing inductance (LMag.) L_RES2.

The COS, placed between OUT and ground, is the equivalent capacitance seen at the driving node (related

to the power switches and to the circuit's distributed capacitors).

Let's start calling basic resonance the one concerning the circuit path net COS + L_RES + CRES.

Where L_RES can range between L_RES1 and L_RES2 depending on the Load value.

Assuming the CRES (tens or hundreds nF) value much higher than the COS one (COS is normally in the

range of nF or lower values), we can just consider the series COS + L_RES.

This basic resonance concerns the soft switching operation.

Soft switching.

Still referring to figure 10: The external active switch (QH) is cut off before the current across the induc-

tance reaches its zero (the active time is less than half period), the current is so forced to flow from the

node OUT; initially discharging COS then, as the voltage cross the voltage threshold, throw the free wheel-

ing diode DL.

After a short time (

≈ 300ns), when the Drain-Source voltage of QL is close to zero, the inherent gate driver

will be activated confirming its "ON" state.

The same will arises at the second half period: Driving off the active QL produces the commutation of the

node Out thanks to the energy transferred from the coil L_RES to the capacitance Cox and the resulting

activation of the opposite diode DH.

Both the rising and falling edges of the commutation can be considered as portions of the "above defined

basic resonance. In Fig.12 it can be observed the portion of sine shaped of the Out node commutations.

LEXT

LSTRAY

LMAGN

DRIVER-H

LOAD

OUT

+HV-BUS

H.B. RESONANCES

L-RES1

L-RES2

COS

CRES

DRIVER-L

D03IN1431

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